The present invention relates to a proton conductive solid electrolyte with high ion conductivity or a hydroxide ion conductive solid electrolyte with high ion conductivity that is applicable to a fuel cell or the like, and an electrochemical system such as a fuel cell using the solid electrolyte with high ion conductivity.
Conventionally, an electrolytic device is realized such as a fuel cell, a dehumidifier, or an electrolytic hydrogen producing device, as an electrochemical system using a proton conductive solid electrolyte. For example, in a solid polymer fuel cell, current flows and electric energy is obtained in accordance with an electrochemical oxidative reaction of hydrogen supplied to a negative electrode that is indicated by the following formula (1), an electrochemical reduction of oxygen supplied to a positive electrode that is indicated by a formula (2), and a reaction based on proton motion in the electrolyte between the positive electrode and the negative electrode.H2→2H++2e−  (1)1/2O2+2H++2e−→H2O  (2)
It is known a fuel cell using a methanol or the like instead of hydrogen as a fuel supplied to the negative electrode. Also in this case, the reaction is carried out in which the fuel is electrochemically oxidized at the negative electrode to release proton, in a similar manner. Therefore, It is possible to operate by using the proton conductive solid electrolyte.
For example of the electrolytic device, the electrolytic hydrogen producing device is realized. The electrolytic hydrogen producing device produces hydrogen on the basis of a reaction inverse to the reaction described in conjunction with the formulas (1) and (2) in the fuel cell. Inasmuch as it is possible to obtain high purity hydrogen in on-site by using only water and electric power in the electrolytic hydrogen producing device, it is unnecessary to have a hydrogen gas cylinder. In addition, it is possible to easily carry out electrolysis by introduction of-pure water having no electrolyte solute, owing to employ the solid electrolyte. Also in paper industry, a similar system is attempted to manufacture hydrogen peroxide for bleach by the electrolytic method using the following formula (3), (referring to a non-patent publication 1).O2+H2O+2e−→HO2−+OH−  (3)
The dehumidifier has a structure in which the proton conductive solid electrolyte film is sandwiched between the positive electrode and the negative electrode, in a manner similar to the fuel cell or the hydrogen producing device. When a voltage is applied between the positive electrode and the negative electrode, water is split into proton and oxygen at the positive electrode in accordance with the reaction indicated by the following formula (4). The proton moves through the solid electrolyte to the negative electrode to be subjected to a reaction indicated by a formula (5). As a result, the union of the proton and the oxygen of air forms water. As a result of these reactions, water moves from the positive electrode to the negative electrode so that dehumidification is carried out in the positive electrode.H2O→1/2O2+2H++2e−  (4)1/2O2+2H++2e−→H2O  (5)
It is also possible to split water and to eliminate moisture, using the principle of operation that is similar to the electrolytic hydrogen producing device. Proposal is made as regards an air conditioner combined with a moisture evaporating cold blast device (referring to non-patent publication 2).
In any one of the above-mentioned systems, perfluoro sulfonic acid type ion exchange membrane represented by Nafion is used as the solid electrolyte. In addition, various kinds of sensors, electrochromic device or the like are essentially a system based on a principal of operation similar to the above-mentioned. Inasmuch as these systems are driven when the proton moves in the electrolyte between a pair of positive and negative electrodes which carry out reduction and oxidation, respectively, it is possible to use the proton conductive solid electrolyte. At present, experimental study is carried out with respect to these systems using proton conductive solid electrolytes.
For a hydrogen sensor, variation of electrode potential based on the concentration of hydrogen is utilized. Proposal is made in the hydrogen sensor about using, for example, the solid electrolyte composed almost exclusively of polyvinyl alcohol, as the electrolyte (referring to non-patent publication 3). Furthermore, it is also possible to be applied to a humidity sensor, using the variation of electrode potential or ion conductivity.
When an electric field is applied to the negative electrode of the electrochromic device by using WO3 or the like, the electrochromic device makes a color on the basis of reaction indicated by the following formula (6) and can be used as a displaying device or a lightproof glass. In this system, proposal is made about Sn (HPO4) H2O which is inorganic compound, as the solid electrolyte (referring to non-patent publication 4).WO3+xH−+xe−→HxWO3(coloring)  (6)
In addition, there are a primary battery, a secondary battery, an optical switch, and an electrolyzed water producing apparatus, as the electrochemical system which operates by using the proton conductive solid electrolyte in principal. For example, in a nickel hydride battery of the secondary battery, a hydrogen storing alloy is used as the negative electrode, a nickel hydroxide is used as the positive electrode, and an alkali electrolytic solution is used as the electrolytic solution. As indicated by formulas (7) and (8), the electrochemical reduction and oxidation occur with respect to the proton in the negative electrode, and hydrogen is stored in the hydrogen storing alloy, on charge and discharge.(charge)H2O+e−→H(storing)+OH−  (7)(discharge)H(storing)+OH−→H2O+e−  (8)
As indicated by formulas (9) and (10), the electrochemical oxidation and reduction occur with respect to the nickel hydroxide.(charge)Ni(OH)2+OH−→NiOOH+H2O+e−  (9)(discharge)NiOOH+H2O+e−→Ni(OH)2+OH−  (10)
The charge and discharge reaction holds in the battery while the proton or the hydroxide ion moves in the electrolyte. Although it is possible to use the proton conductive solid electrolyte in principal, the alkali electrolytic solution is used in the prior art.
Proposal is made in the optical switch about using yttrium as the negative electrode (referring to non-patent publication 5). When supplied with the electric field, yttrium is hydrogenated as indicated by formula (11) to allow the light to pass therethrough. As a result, it is possible to switch between the light transmission and the non-light transmission by electric field. Although it is possible to use the proton conductive solid electrolyte in principal in this system, the alkali electrolytic solution is used in the prior art.Y+3/2H2O+3e−→YH3+3OH−  (11)
The electrolyzed water is water which is produced by the electrolyzing reaction. Although availability is different between the reduction side and the oxidation side, the electrolyzed water has availability in a healthy effect, a bactericidal action, a detergent action, and a growth of farm products. It is possible to use the electrolyzed water in drinking water, food service water, detergent water, agricultural water or the like. The electrolyzing reaction is promoted when the water has the electrolyte. When the electrolyte is dissolved in water, it is often necessary to remove the electrolyte from the water on using the water. When the solid electrolyte is used as the electrolyte, it is unnecessary to remove the solid electrolyte from the water.
However, there is a problem in which the perfluoro sulfonic acid type electrolyte is expensive on account of complexity of manufacturing process although the perfluoro sulfonic acid type electrolyte is used in the fuel cell, the electrolysis type hydrogen producing device, the dehumidifier, or the like which is realized in the prior art, as the electrochemical system using the proton conductive solid electrolyte. By the economies of mass production, it is expected that a low-priced electrolyte is manufactured. However, there is limitation of the low-price. It is desired that a cheap alternate member appears presently.
By the way, the proton travels at a high speed by operation of water included in the solid in the proton conductive solid electrolyte which operates at an ordinary temperature. As a result, it is necessary for the alternate member to have sufficient water absorption. More particularly, the proton conductive solid electrolyte must further have water resistance inasmuch as most proton conductive solid electrolyte is used in a damp environment. In the conventional perfluoro sulfonic type electrolyte, the water, which is absorbed in the highly hydrophilic sulfonic acid group, transports the ion, and the poly-fluoro ethylene bone structure has a role for maintaining water resistance, chemical stability, high temperature durability.
The polyvinyl alcohol is an example of hydrocarbon polymers that is highly hydrophilic and cheap. It is possible to use the material having the proton conductivity by mixing phosphoric acid into the polyvinyl alcohol, as the hydrogen sensor or the like. Although the proton travels at high speed inasmuch as the polyvinyl alcohol has the high water absorption, there is a problem in which a material stability is low in the damp environment because the polyvinyl alcohol is soluble in water.
An inorganic hydrated compound is known as another material having a highly hydrophilic property and a high durability and water resistance. For example, the hydrated glass of P2O5—ZrO2—SiO2 that is produced by the sol-gel process absorbs much water to have a high proton conductivity and does not dissolve in water. The hydrated glass has a high stability in a high temperature that is specific to the inorganic compound (referring to non-patent publication 6).
However, each of the inorganic hydrated compounds is brittle as a common weak point. More particularly, it is difficult to make each of the inorganic compounds to a thin film which is required in use of the solid electrolyte. Furthermore, an expensive metal alcoxide is used as a material in the sol-gel process and also it is difficult to reduce an equipment cost for manufacturing, because organic solvent such as alcohol is used in the sol-gel process. Although it is possible to apply powder of the above-mentioned Sn(HPO4)·H2O which is used in the electrochromic device, it is difficult to make the above-mentioned Sn (HPO4)·H2O to a film having a high intensity and a gas diffusion restrictive function which are required in the fuel cell or the like. In addition, molybdophosphoric acid and tungstophosphoric acid are reported as inorganic compounds each of which has the high conductivity (referring to non-patent publication 7). The molybdophosphoric acid is represented by composition formula of H3MoPO40·29H2O. The tungstophosphoric acid is represented by composition formula of H3WPO40·29H2O. Furthermore, each of ZrO2·nH2O, SnO2·H2O, and Ce(HPO4)2 is reported as the inorganic compound having the high conductivity (referring to non-patent publication 8). It is difficult to make the thin film even though using the molybdophosphoric acid, the tungstophosphoric acid, or other materials.
Means is proposed which combines the hydrophilic organic polymer with the inorganic compound, as the method of overcoming the defects of the hydrophilic organic polymer and the inorganic compound. For example, a proton conductive material is proposed which is obtained by chemically bonding silicide to polyethylene oxide in a nano-technology (referring to patent publication 1). Although the polyethylene oxide is a cheap and hydrophilic organic polymer similar to the polyvinyl alcohol, the polyethylene oxide dissolves in water when it is used singly. But, it is possible to make the polyethylene oxide have the water resistance when the polyethylene oxide is combined with silicide by using the sol-gel process. As a result, it is possible to obtain the material having a good intensity in a high temperature. However, it is difficult to obtain a compound material by other methods instead of the sol-gel process. Disclosure is not made as regards the other methods. Therefore, there is a problem in which it is difficult to reduce the material cost and the manufacturing cost. In addition, proposal is made about an ion conductive material which is obtained by combining the organic compound such as polyethylene oxide with the inorganic compound such as the silicide and a proton conductive adding agent such as the tungstophosphoric acid or the molybdophosphoric acid (referring to patent publication 2). However, disclosure is made in patent publication 2 about only combining method based on the sol-gel process.
All of the above-mentioned conventional solid electrolytes are acid, respectively. The material, which is used as the electrode or other system configuring material, is limited to a material having an acid resistance, such as a noble metal. As a result, it is difficult to reduce the cost of entire system. When the solid electrolyte is acid, it is difficult to use the acid electrolyte in the primary battery, the secondary battery and the optical switch, because the electrode and the active material are degraded by the solid electrolyte. Furthermore, the alkali liquid state electrolyte may leak which is used in the conventional primary battery and the conventional secondary battery.
On the other hand, proposal is made about a gel electrolyte in which an aqueous electrolyte such as an alkali electrolytic solution is gelated by polyacrylic acid (referring to non-patent publication 9). Although the gel electrolyte is not the solid electrolyte substantially, the gel electrolyte has a high ion conductivity which is approximately equal to that of the aqueous electrolyte. Furthermore, the gel electrolyte is cheap and it is possible to prevent leakage of the electrolytic solution. However, the gel electrolyte does not have sufficient strength and has a low ability for preventing diffusion of gas or ion. Therefore, the gel electrolyte is used only in limited applications.
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